Southwest Airlines faced a fourth day of disruptions Monday — after canceling hundreds of flights over the weekend — a sign of airlines’ struggles to capitalize on a growing appetite for travel amid a pandemic that has scrambled the industry and left some carriers stretched thin.

Southwest said it was working to return flight operations to normal as it dealt with a backlog of passengers from more than 2,000 canceled flights and thousands more that were delayed since Friday. The airline on Monday canceled 363 flights — about 10 percent of scheduled departures — while more than 1,300 were delayed, according to flight-tracking site FlightAware.

The disruptions inconvenienced, angered and stranded tens of thousands of passengers. The cancellations and delays Monday at the nation’s fourth-largest domestic carrier were a costly extension of disruptions that stressed travel during the long holiday weekend.

The latest problems mirrored disruptions that travelers encountered over the summer in a scenario that has become increasingly common as the industry recovers from the pandemic. Airlines have shed staff during the pandemic, and analysts say scheduling problems can more easily snowball, leading to mass flight cancellations.

Southwest reiterated Monday that inclement weather and air traffic control disruptions in Florida on Friday triggered the problems. Federal regulators said that air traffic control staffing shortages caused delays out of Florida but that airlines generally are experiencing operational issues because of their own staffing and aircraft issues.

Southwest on social media asked for patience as airport lines grew and passengers sought to rebook their flights. The carrier is trying to recover after canceling more than 1,000 Sunday departures — nearly 30 percent of its flights — which followed 800 cancellations Saturday.

As of Monday, many of the flights that were canceled or delayed continued to involve large cities — including Chicago, Denver and Baltimore — where many of the airline’s transfers occur. Many flights also were canceled in Dallas, where the carrier has its headquarters.

“We have a closer to normal operation today, navigating some new weather across our system,” the airline said in a statement Monday. “We spent the weekend working to recover from the high number of displaced Crews, aircraft and Customers.”

Passengers and airline observers questioned Southwest’s explanation for the widespread disruptions, noting that other airlines have seen no significant effects from weather or air traffic control challenges.

FlightAware said that as of Monday morning, 38 percent of Southwest’s flights since Friday had been delayed, at least double the rate of its biggest competitors. For Delta Air Lines and American Airlines, that figure was 19 percent, while 14.5 percent of United Airlines flights were delayed.

The flight woes ignited claims from conservatives, apparently based on a coincidence in timing, that issues were the result of a protest of Southwest’s vaccine mandate. The travel problems came days after Southwest announced it was requiring employees to receive vaccinations against the coronavirus — a mandate that some airlines have had for weeks without experiencing similar issues.

The carrier said last week that employees will have until Dec. 8 to provide proof of vaccination or face the possibility of losing their jobs, with allowances for religious, disability or medical exemptions. President Biden last month announced that private companies with 100 or more employees would be required to ensure that their workers were vaccinated or implement weekly testing programs.

Although U.S. airlines fall into that category, many also are government contractors who must meet a Dec. 8 deadline for vaccinations. There is no testing option for contractors.

“Joe Biden’s illegal vaccine mandate at work!” Sen. Ted Cruz (R-Tex.) tweeted Sunday about Southwest’s woes. “Suddenly, we’re short on pilots & air traffic controllers.”

Southwest on Monday countered reports about employee protests, saying that “the weekend challenges were not a result of Employee demonstrations, as some have reported.” The Southwest Airlines Pilots Association also rejected speculation of pilot protests as “false claims.”

Denver Airport saw massive lines and congestion over the weekend of Oct. 10, after Southwest Airlines canceled at least 1,800 flights and delayed 1,500 more. (@rylacy via Spectee)

“I can say with certainty that there are no work slowdowns or sickouts either related to the recent mandatory vaccine mandate or otherwise,” association president Casey Murray said in a statement.

Stranded passengers took to social media to voice frustrations, searching for answers from the airline as many struggled with long waits at the airline counters and on a customer service phone line. Rebooking, some said, was a nightmare.

Erin Cadwalader had hatched a plan to surprise her sister this weekend, heading from her house in Maryland on Friday to meet at their aunt’s house in Florida.

“My sister loves surprises, so I was really excited to have the opportunity to pull something off like this,” she said. Their mother — her aunt’s sister — died last May, and it was to be the first time the two women had seen their aunt since the pandemic began.

Cadwalader, 40, had picked up food at Reagan National Airport and ordered a drink when she got a text message indicating her flight had been canceled. Not knowing whether she would be able to fly, she revealed her plan to her sister.

Cadwalader got a flight from Baltimore-Washington International Marshall Airport on Saturday afternoon but woke up at 4:30 a.m. Monday to news that her return flight had been canceled. She had been booked onto a Tuesday night flight but decided to spend $250 on a flight with American.

“I don’t honestly have confidence that Southwest won’t cancel my flight tomorrow night as well, given how untransparent they’ve been about what’s causing these problems,” Cadwalader said.

Kurt Ebenhoch, executive director of the Air Travel Fairness Coalition, which advocates for air travel consumers, called Southwest’s handling of the crisis “disingenuous,” noting the company’s homepage Monday offered no information about operational problems to customers.

“By blaming air traffic and weather, Southwest is just not being honest with their customers … If that was the cause, other airlines would be impacted at the same magnitude, and that’s not happening,” he said. “The data speaks for itself.”

Murray blamed the weekend’s meltdown on airline management’s poor planning, questioning Southwest’s claims that issues at the Jacksonville Air Control Center and weather in the Southeast are to blame.

“What was a minor temporary event for other carriers devastated Southwest Airlines because our operation has become brittle and subject to massive failures under the slightest pressure,” he said.

Airlines received tens of billions of dollars in federal aid to ensure they kept staff on the payroll during the pandemic, which slashed demand for flights. Nevertheless, carriers have slimmed down, shedding thousands of jobs as employees have retired or left voluntarily. Southwest had about 62,000 employees as the pandemic took hold, but employed 54,500 workers in August, according to Transportation Department figures.

Staffing shortages were blamed for flight cancellations and delays at Southwest over the summer, prompting the airline to reduce its schedule to avoid more disruptions.

Delays and cancellations have also hurt several other airlines. In August, bad weather set off a cascade of issues at low-cost Spirit Airlines that left it reeling for a week. American Airlines blamed storms in Texas for the delays and cancellations over the summer.

The Transport Workers Union of America, AFL-CIO, which represents airline employees, warned in July that staffing shortages were affecting morale and that some workers reported feeling exhausted and overwhelmed.

Experts say staff reductions during the pandemic have left airlines vulnerable to disruption as travel demand has grown.

Southwest is especially vulnerable because unlike other airlines that base crews and aircraft in central locations, it has a “point-to-point” system, with aircraft and crews scattered in more places. That setup makes it more difficult to move crews after a disruptive event.

The delays and cancellations at Southwest are an indication of a growing problem facing airlines that have been stretched thin for months during a travel rebound, said Max Leitschuh, a senior transportation analyst at Crisis24, a global risk management firm.

“Pretty much every major U.S. airline out there is really overstretched, especially the pilots and flight attendants. And adding those types of employees is not a very quick task,” he said. “All it takes is one little issue and that creates a ripple effect throughout the airline. And if the airline doesn’t handle it right, that can cause their operations to basically melt down for days.”

Lori Aratani contributed to this report.